1. Field of the Invention
This invention relates generally to optical transmission systems and, more particularly, to polarization multiplexed optical transmission systems.
2. Description of the Related Art
In optical polarization multiplexed transmission systems, two optical signals each having a common wavelength, but one of two orthogonal polarizations, may be independently modulated, and then multiplexed together for transmission over a network infrastructure, over a fiber optical cable, for example. Due to their mutual orthogonality, the optical signals can be differentiated from each other at an optical receiver, located at a terminal node where a client signal interfaces to a network infrastructure, for example. Polarization multiplexing, therefore, potentially doubles the transmission capacity for each wavelength channel.
One requirement for polarization demultiplexing the two optical signals, with acceptable tolerance to noise, is for the two polarizations of the two optical signals to be perfectly orthogonal at the point of multiplexing, for example in an optical transmitter. If the two polarizations are not orthogonal, then a portion of each of the two optical signals will overlap in both frequency and polarization, and may not be distinguished from each other at the receiver or terminal point. The intermixing of one optical signal on top of a second leads to noise on the second channel, and hence data bit errors. The polarization states associated with optical signals propagating in a transmission system are preferably well maintained from the point of origin, e.g. an output of a light source, through one or more optical elements, to a polarization beam combiner. Such one or more optical elements may include, but are not limited to, modulation elements, wavelength multiplexing elements, as well as passive elements such as the various waveguide structures which transport the optical signals from one element to another, as part of a photonic integrated circuit (PIC) for example. However, waveguide attributes such as waveguide imperfections and deviations in the fabrication process which may impart some randomness in the fabrication structure, waveguide surface roughness scattering, waveguide junctions, and waveguide bends may serve to re-orient the polarization state as the optical signal propagates in a photonic integrated circuit (PIC).
There is a need to maintain, or mitigate changes in, the polarization state of an optical signal propagating in a photonic integrated circuit. Further, there is a need to maintain, or mitigate changes in, the polarization state of an optical signal propagating from one photonic integrated circuit to another photonic integrated circuit.